1. Field of the Invention
The invention relates to an electroluminescent display device and the method of driving the same, and more particularly to a TFT electricity reset process utilized in an electroluminescent display device and the driving method thereof.
2. Description of the Related Art
As an electric current driven device, the organic light emitting diode has a property that it emits light having intensity in proposition to the current through the light emitting diode. In general, Low Temperature Poly Silicon Thin Film Transistor (LTPS-TFT) and Amorphous Silicon Thin Film Transistor (a-Si TFT) are most popular technology used to fabricate the active element of the organic light emitting diode. In practice, the Poly Silicon technology is often utilized. However, due to less mask processes, lower temperature, and low cost, developing a-Si TFT technology is a tendency. After a long term use, due to some material characteristics and circuit design, the active element (no matter LTPS TFT or a-Si TFT) of the organic light emitting diode will suffer from raised threshold voltage and lowered turn-on current. It is especially true for the a-Si TFT technology.
When a-Si TFT is used as an active element of a electroluminescent display panel, and the active element is turned on for conducting current, a large current will flow through the channel of the a-Si TFT. Due to the foregoing scenario, it tends to trap the electron of the current in the gate dielectric, results in raise of the threshold voltage of the a-Si TFT, as well as drop of turn-on current through the a-Si TFT. Subsequently, it descends—the luminance of the organic light emitting diode, and reduces the life of the display panel.
Due to the problems mentioned above, when the a-Si TFT is utilized in the electroluminescent display panel, its sequence of driving is different from that of the electroluminescent display panel utilizing LTPS TFT as active element. As widely used in electroluminescent display panel, the LTPS TFT acts as active element, and it is necessary to continue refreshing the display panel. However, when it comes to a-Si TFT, in addition to refreshing the display panel, a “TFT electricity reset sequence” is made possible, and the life of the a-Si TFT used in the electroluminescent display is extended.
In FIG. 1A, it schematically illustrates the circuit diagram of the pixel matrix of the active matrix type electroluminescent display device. As shown in FIG. 1A, the display panel includes M scan lines, N data lines, and M times N (M×N) pixels, which are used to graphically illustrate an image signal composed of a plurality of frames. According to FIG. 1A, the OLED (organic light emitting diode) D (1,1) in the pixel P (1,1) is driven by both TFT Ta (1,1) (thin film transistor) and TFT Tb (1,1), wherein the source and gate of Ta (1,1) are coupled to the data line Data (1) and scan line Scan (1), respectively.
FIG. 1B is a timing chart of a plurality of driving pulse sequences, which in combination with FIG. 1A can be used to explain the operation of a traditional active matrix type electroluminescent display device. As shown in FIG. 1B, the period from the beginning of a specific scan line selection to the beginning of the next selection of the foregoing specific scan line is defined as display period I, it is also the time interval required to show a frame on the display panel. The display panel of the active matrix type electroluminescent display device in the related art can be driven by the method including the step of: subsequently scanning each row of pixel P, i.e., subsequently applying a positive pulse to scan lines, Scan (1) to Scan (M), thus each of the transistor Ta in each row of pixel P is turned on; when transistor is on, a data signal is fed to a corresponding data line, one of Data (1) to Data (N), responding to a designated pixel P. Accordingly, the designated pixel, which is intended to be lightened up, corresponding to a specific address is selected and fed with the data signal. In addition, the different voltage levels in the data signal represent different luminance of pixel P.
According to the driving method in the related art, when a pixel is lightened up, the voltage of the capacitor C corresponding to the pixel must be kept at a high level during the whole display period, thus the gate of the corresponding transistor Tb is always kept at the high voltage level, and there is always a current flow through the transistor Tb, which results in the transistor Tb's threshold voltage shift. In detail, when the transistor Tb is formed of a-Si, there will be a gate insulator layer covering the gate of the transistor Tb. As the gate of the transistor Tb keeps at high voltage level, the electron in the channel layer of the transistor will be trapped in the gate insulating layer, which in general, is formed of silicon nitride (SiNX). Thus the voltage level, required to turn on the transistor Tb, on the gate is raised, i.e., the threshold voltage of the transistor Tb is raised. In addition, because the voltage level applied on the transistor Tb from the capacitor C is fixed, the raise of the threshold voltage of the transistor Tb will result in a decline in the current flow through the transistor Tb, thus obscuring the organic light emitting diode (OLED). In a long term, not only the luminance of the OLED will be decreased, but also some more serious problems will happen to transistor Tb.
In light of the problems mentioned above, one kind of related art use alternative method to drive the active matrix type display device with its circuit configuration unchanged. As shown in FIG. 1C, it illustrates the timing chart of a plurality of driving pulse sequences, wherein the period required to display a frame on the display device is defined as display period I, which includes a first period IA and a second period IB. At first, within the first period IA, subsequently apply a first pulse A1 to the scan lines, Scan (1) to Scan (M), and apply a date signal to data lines, Data (1) to Data (N). Then apply a second pulse B1 to the scan lines, Scan (1) to Scan (M), to turn on all corresponding transistors Ta, followed by applying a first voltage signal Vb to data lines, Data (1) to Data (N), within the second period IB, thus turning off corresponding transistors Tb. So the time interval which the transistors Tb are turned on is reduce to one half when compared with the previous example illustrated in FIG. 1B. It is the reason why the driving method illustrated in FIG. 1C can suppress the threshold voltage of the transistors Tb from shifting.
Because the traditional electroluminescent display device using a-Si TFT is designed to perform the TFT electricity reset sequence when the screen (display panel) being set black. From activating the first scan line to black-screen-setting, the foregoing time interval is different from the following time interval, from activating the last scan line to black-screen-setting. Specifically, because the first scan line, in the timeline, is the first shown on screen, the corresponding pixels continue emitting light from the beginning. After the voltage levels in the data signal have been applied to the corresponding last scan line, all the pixels, including the pixels from the first scan line to the last scan line, on the screen will be processed by the TFT electricity reset sequence. So the following phenomenon is resulted—the pixels of the first scan line is obviously brighter, and the pixels of the last scan line is apparently darker.